1. Field of the Invention
This invention relates to a vibration wave motor which is frictionally driven by a travelling vibration wave generated in an elastic vibration member.
2. Related Background Art
Vibration wave motors for frictionally driving a moving member by utilizing a travelling vibration wave are proposed in, e.g., U.S. Pat. Nos. 4,580,073 and 4,484,099. These motors have been commercially available in recent years. The principle of operation of the vibration wave motor will be described below.
Two groups each consisting of a plurality of piezoelectric elements are fixed on one surface of a ring-like elastic vibration plate having a circumferential length which is an integer multiple of a given length .lambda. to constitute a stator. These piezoelectric elements are normally arranged at .lambda./2 pitches in each group and alternately have opposite polarities. The piezoelectric elements in the groups are offset by an odd number multiple of .lambda./4. Electrode films are formed on the piezoelectric elements of the groups, respectively. When an AC voltage is applied to any one of the groups, a standing wave (wavelength: .lambda.) of flexural vibrations is generated throughout the entire circumference of the vibration plate such that antinode positions are located at the central positions of the piezoelectric elements of the group and positions away from the central positions every .lambda./2 intervals, and nodes are located at the central positions between the antinodes. When an AC voltage is applied to the remaining group, a similar standing wave is generated. However, in this case, the positions of the antinodes and the nodes are offset from the first standing wave by .lambda./4. When AC voltages having a positional difference of .pi./2 as a function of time and having the same frequencies are simultaneously applied to both the groups, two standing waves are combined to generate a travelling wave (wavelength:.lambda.) of flexural vibrations in the circumferential direction of the vibration plate. In this case, the respective points on the other surface of the vibration plate having a predetermined thickness are subjected to a kind of elliptical motion. If a ring-like moving member serving as a rotor is brought into tight contact with the other surface of the vibration plate, the moving member receives a circumferential frictional force from the vibration plate and is rotated. The direction of rotation can be reversed by changing a positive phase difference between the AC voltages applied to both the groups into a negative difference, and vice versa. The above description is concerned with the principle of operation of a vibration wave motor of this type.
A driver circuit in a conventional vibration wave motor of this type is proposed in, e.g., Japanese Pat. Laid-Open (Kokai) No. 61-157276, U.S. Pat. No. 4,501,411, and Japanese Pat. Laid-Open (Kokai) No. 59-156169. One vibration detection piezoelectric element is fixed on the other one of the groups of piezoelectric elements (these elements are referred to as driving piezoelectric elements), and a frequency of the AC voltage applied to the driving piezoelectric elements is automatically changed into a resonance frequency in accordance with a detection output from the detection piezoelectric element, thereby improving efficiency of the vibration wave motor.
In the vibration wave motor described above, however, the vibration detection piezoelectric element is fixed at the same spatial phase position as that of one of the groups of driving piezoelectric elements. More specifically, since the central point of the vibration detection piezoelectric element is located at a position offset from the central point of one area of the group of driving piezoelectric elements by a integer multiple of .lambda./2, thus posing the following problems.
First, since the frequency characteristics of standing waves generated upon application of an AC voltage to the groups differ from each other due to the vibration detection piezoelectric element located at the same spatial phase position as that of one of the groups of driving piezoelectric elements, the vibration detection piezoelectric element can detect only a vibration state of the standing wave generated by applying the AC voltage to one group of driving piezoelectric elements.
Second, a time-phase difference between both the standing waves generated upon application of the AC voltages to both the groups and an amplitude of one of the standing waves must be controlled by an open loop due to the first reason. The time-phases of the standing waves are greatly shifted from .pi./2, and a difference between the amplitudes of the standing waves is increased. As a result, the amplitude of the travelling wave greatly varies, thus causing degradation of efficiency and unstable rotation of the motor.